The present invention relates generally to methods and apparatus for regenerating an exhaust after-treatment device and, more particularly, to methods and apparatus for regenerating an exhaust after-treatment device using a two-stage regeneration.
Environment regulations have necessitated the addition of various exhaust after-treatment devices in exhaust after-treatment systems (EATS) for internal combustion engines. For diesel engines, for example, it is now common to include a particle filter (typically referred to as a diesel particulate filter or DPF) in the EATS. Gasoline engines, especially those of the direct injection type where fuel is directly injected in the combustion chamber, may also in the future be equipped with such filters. An EATS may comprise other components, such a NOx reducing devices. In the case of a Diesel engine, a NOx reducing catalyst (typically a selective catalytic reduction catalyst (SCR)) is typically provided downstream of the particle filter and an oxidation catalyst (often referred to as a diesel oxidation catalyst (DOC)) upstream of or forming part of the filter. The oxidation catalyst oxidizes CO and NO in the engine exhaust and converts them to C02 and N02. Different arrangements for providing oxidation catalysts include a DOC with an uncoated DPF (so called CRT systems), DOC with a coated DPF (so called cCRT systems), and a coated DPF.
Exhaust from the engine is ordinarily at a temperature of about 250-350° C. and, at these temperatures, a certain amount of so-called “passive” N02 regeneration of the particle filter 29 can occur wherein collected soot can be oxidized and removed from the filter, particularly when aided by the oxidation catalyst upstream or on the particle filter, via the reaction(s):C+2N02→C02+2NO  (1)and/orC+N02→CO+NO  (2)
At low temperatures (<300° C.) the soot oxidation by N02 shows quite a low rate and therefore the soot loading of the particle filter can increase quite quickly in case of severe duty cycles, where the temperature of the exhaust gas may be bow 200° C. (cold unloaded cycles). Excessive soot on the particle filter can impair functioning of the particle filter or the engine. It sometimes happens that, over a given duty cycle a balance is reached wherein the soot that is collected by the particle filter is burned off at the approximately same rate as it is oxidized by passive regeneration, thereby maintaining the soot loading in the particle filter within acceptable levels. However, on certain duty cycles, soot loading becomes too high and it is necessary to regenerate the particle filter through a specific active regeneration procedure.
In the past, it was typical to perform the regeneration by a so-called “active” 02 regeneration. Active 02 regeneration occurs via the reaction(s):C+02→C02  (3)and/or2C+02-2C0  (4)
Active 02-based regeneration systems raise the temperature of the reactants, through a variety of methods, in order to establish and sustain an 02/soot reaction. During active 02-based regenerations, it is believed that substantially all soot removal is via reaction with 02.
A problem with 02 regeneration is that it is performed at high temperatures, usually around 600-625° C. for a catalyzed particle filter. The reaction is highly exothermal and it is generally considered that there is an unacceptable risk of “runaway” or uncontrolled regeneration at temperatures above 550° C. if soot loading levels of the particle filter are too high. For a given arrangement, one can determine a safe high temperature regeneration soot loading level, below which high temperature regeneration is assumed to be safe. It is then considered that performing a high temperature regeneration, when the particle filter is loaded at more than the safe high temperature soot loading level, involves a risk of uncontrolled regeneration. In some frequently used EATS arrangements, the safe high temperature soot loading level can be considered to be above about 2-8 grams of soot per liter of filter (expressed herein as gC/l filter). (The particular loading levels will vary due to various factors, such as filter type) Accordingly, because of the danger of uncontrolled regeneration at these soot loading levels above, an active 02 regeneration would be performed when soot loading levels approached that level, even though the particle filter and other components may have been capable of functioning adequately at much higher soot loading levels. Each 02 regeneration involves cycling the temperature of the filter or the exhaust gas at the particle filter to around 600-625° C. or higher. This cycling tends to increase wear on the particle filter, and also to involve substantial use of energy to heat the exhaust gas or filter.
In addition to regeneration of the particle filter, it is occasionally necessary to “de-poison” components in the EATS such as the oxidation catalyst or the NOx reduction catalyst, or even other NOx reducing devices such as so-called NOx traps. The catalyst poisons are typically removed by applying high thermal processes. In some specific cases (as S poisoning), the regeneration (sometimes referred to as “de-poisoning”) requires quite high temperatures, as for example around 600° C. for a Cu-zeolite SCR catalyst. In an after-treatment system with an SCR downstream of the particle filter, reaching 600° C. in the SCR is only possible if temperatures are greater than 600° C. at the particle filter. At that temperature, while the SCR is de-sulfated, the filter is also regenerated (from soot). The regeneration at that temperature will be an active 02 regeneration and therefore it is further necessary to pay special attention to maximum soot loadings of the particle filter. Because it was necessary to keep soot loading below the level at which uncontrolled regeneration could occur if an 02 regeneration were performed, regeneration of the particle filter and/or de-sulfurization of the SCR would be quite frequent in order to prevent high exothermal effect in the filter. This frequent regeneration led to a higher fuel penalty and catalyst aging.
Because of the risk of uncontrolled regeneration, when soot loading levels are above the safe high temperature soot loading level at which there is considered to be an unacceptable risk of uncontrolled regeneration, and even when soot loading levels are below that level, 02 regeneration is usually only performed on vehicle-mounted engine arrangements when the vehicle is parked. This means that the vehicle is periodically taken out of service to perform the 02 regeneration.
It has recently been discovered that N02 regeneration with enhanced effective N02 supply (hereinafter referred to as “enhanced N02 regeneration”) occurs in an intermediate temperature range between the range of normal exhaust temperature at which passive N02 regeneration tends to occur and the temperature range of active 02 regeneration, such as temperatures in a range of about 420-550° C. It has been theorized in U.S. Patent Application Publications US2011/0000190A1 and US2010/0326055A1, which are incorporated by reference, that, at temperatures of 450-550° C., less than two thirds and possibly less than half of the soot is removed by reaction with 02, while most of the rest of the soot is believed to react with N02.
U.S. Patent Application Publications US2011/0000190A1 and US2010/0326055A1 introduce the concept of an “effective N02 supply”, which effective supply will be enhanced to increase its soot removal efficacy relative to the efficacy that would be expected during conventional N02-based regeneration, even if the N02/NO ratio and therefore the equilibrium-limited N02 supply decreases. The effective N02 supply was defined as the amount of N02 that participates in soot oxidation. The participating N02 can either come directly from the equilibrium-limited N02 supply, NO oxidized in the catalyzed DPF, or from NO recycling. The concept of the soot removal capacity of the N02 reactant was also introduced. Even though enhanced N02 regeneration can cause the equilibrium-limited N02 supply to decrease, it can at the same time greatly increase the effective N02 supply, thereby increasing the soot removal capacity of the equilibrium-limited N02 supply, resulting in a significantly higher soot oxidation rate. Conditions can be controlled so that, even though a lesser quantity of N02 is supplied to the particle filter than under conventional conditions, the rate at which NO is converted to N02 and that N02 reacts with soot within the DPF is greater than under the conventional conditions where, ordinarily, a larger, equilibrium-limited quantity of N02 would have been supplied to the DPF. It is theorized that the NO is effectively “recycled”, usually more than once, through a catalytic reaction to form N02, which in turn, reacts with soot, again forming NO which is catalytically reacted, etc. Thus, a particular quantity of NOx in the engine exhaust can, under conditions of an enhanced N02 regeneration, be effective to oxidize more soot than an equilibrium-limited N02 supply.
U.S. Patent Application Publications US2011/0000190A1 and US2010/0326055A1 explain that, in N02-based regeneration testing, a measurement of N02 efficiency, which is related to the reaction stoichiometry of N02 and C, is introduced to evaluate the effectiveness of a particular method. The N02 efficiency is expressly defined as the mass of C removed from the DPF divided by the mass of N02 provided to the DPF, determined over a time period that is significant with respect to, but not exceeding, the time required to effectively regenerate a substantially full DPF. Conventional wisdom for conventional N02-based regeneration dictated that N02 efficiency would not significantly exceed 12.01 gC/46.01 gN02=−0.26 gC/gN02. The unit “gC” is the mass of soot removed from the DPF and the unit “gN02” is the mass of the accumulated equilibrium-limited N02 supply. Even more so, it was assumed that at elevated temperatures (near or just beyond the NO—N02 conversion plateau) total N02-based soot oxidation activity would fall significantly as the increasingly smaller equilibrium-limited N02 supply would not be able to take advantage of the increased temperatures. In other words, increasing temperatures would simply lower N02 supply and result in a more diffusion-limited reaction, therefore lowering the reaction rate, and thereby achieving lower total soot removal. Conventional passive N02-based regenerations have N02 efficiencies considerably less than 0.52 gC/glM02, and more commonly less than 0.26 gC/gN02, over a time period that is significant with respect to, but not exceeding the time required to regenerate a substantially full DPF.
U.S. Patent Application Publications US20110000190A1 and US2010/0326055A1 explain that, by actively increasing reactant temperature, significantly better soot removal results can be achieved than with conventional N02-based regeneration techniques, with N02 efficiencies of well above 0.52 gC/gN02, ordinarily multiples above that efficiency. This is achieved by increasing the soot removal capacity of the N02, with the objective of enhancing the effective N02 supply (and not necessarily the equilibrium-limited N02 supply). While not wishing to be bound by theory, it is believed that the mechanism whereby the soot removal capacity of the N02 is increased is the NO recycling mechanism. Within a catalyzed DPF given sufficiently long residence times and sufficiently high temperatures, an N02 molecule which has reacted with soot and formed an NO molecule may then be recycled back into N02, which may in turn participate in another soot oxidation reaction. This process may repeat itself as many times as the residence time, kinetic reaction rates of the soot oxidation and the NO oxidation reactions, soot availability, oxygen availability, and catalyst availability will allow.
According to an aspect of the present invention, a process is provided for regenerating an exhaust gas after-treatment device adapted to be fitted in an exhaust line of an internal combustion engine, wherein the exhaust gas after-treatment device is one of a particle filter and/or of a NOx reducing catalyst, characterized in that the process comprising the steps of:
a) setting the temperature of the exhaust gases at the particle filter within a first temperature range, preferably comprised between 420 and 550 degrees Celsius;
b) maintaining the temperature of the exhaust gases at the particle filter at the first temperature range during a first period of time;
c) after the first period of time, further gradually increasing the temperature at the particle filter to a second temperature range which is over 550° degrees Celsius.
The inventors have discovered that, among other things, by performing the regeneration in two stages where a first stage, which can be called active N02 regeneration process, is performed at intermediate temperatures below those at which there is a substantial risk of uncontrolled regeneration if high-temperature, active 02 regeneration were to be performed, maximum soot loading levels of a DPF can be increased above the safe high temperature regeneration soot loading level above which it is no longer considered to be safe to perform an active 02 regeneration of the particle filter. For example, some filters and engine arrangements will continue to function adequately at soot loading levels of the particle filter up to about 1.5 to 2 times higher than the safe high temperature regeneration soot loading level, however, maximum loading levels may vary for a variety of different factors.) Thus, this first stage of a regeneration process facilitates less frequent regenerations, and less frequent interruption of use of a vehicle including the engine and EATS. Nevertheless, in a second stage, the regeneration process involves an active 02 regeneration, performed at high temperatures, typically above 550° C. and preferably above 600° C., process by which remaining soot in the particle filter is substantially completely remove in a limited amount of time. In case a NOx reducing device, such as a NOx reducing catalyst, is located downstream of the particle filter, the second stage of the process allows simultaneous de-poisoning of the NOx reducing device. The two stage process according to the invention therefore allows less frequent regenerations while nevertheless optimizing the time needed to achieve a substantially complete regeneration of the particle filter where the soot loading level is brought back to a minimum level.
According to other aspects of the invention:                during step c), the temperature at the particle filter can be increased at a controlled rate of temperature increase over time, so as to further increase safety of the regeneration by further safeguarding against the risk of a run-away reaction in the particle filter, which could be due to uncontrolled oxidation of excessive amounts of soot. Since the temperature at the particle filter increases over time, it is equivalent to consider a controlled rate of temperature increase over time or a controlled rate of temperature increase increases over the temperature itself.        during step c), the temperature at the particle filter is increased at a controlled rate of temperature increase over time which is controlled as a function of a soot loading determination of the particulate filter, so that a better compromise can be achieved between the safety of the regeneration and its duration. Especially the temperature at the particle filter can be chosen to be increased at a rate of temperature increase over time which decreases over time and/or over the instant temperature at the particle filter.        during step c), the temperature at the particle filter may be increased at a variable rate of temperature increase over time, the variation of which is controlled as a function of the soot loading determination of the particle filter, for example at the end of the first period of time. This allows further optimization of the safety/duration compromise. For example, during step c), the temperature at the particle filter may be increased in at least two sub-steps:        in a first sub-step c1) at a first rate of temperature increase over time;        in a second sub-step c2) at a second rate of temperature increase over time, where the second rate is preferably lower than the first rate.        In some embodiments, the second rate of temperature increase over time during the second sub-step c2) is adjusted at a higher value when soot loading of the particle filter is estimated at a lower value, as an implementation of a variable rate of temperature increase over time.        In some embodiments, the soot loading of the particle filter can be an estimated soot loading, for example by using a soot loading model for the particle filter, because precise measurement of the soot loading might be difficult and/or expensive to implement in an operating environment. One parameter for estimating the soot loading may be the duration of the first period of time of step b). To have a more precise estimation, the soot loading may be estimated depending on the duration of the first period of time of step b) and depending on an estimated soot loading at step a). Alternatively, or in combination, the soot loading maybe estimated depending on a measured pressure difference between at an inlet and at an outlet the particle filter. Alternatively, or in combination, the soot loading may be estimated using an engine out soot emission model that estimates the soot emitted by engine as a function of engine operating parameters, and using a soot regeneration model that estimates soot oxidation in the particle filter based on operating conditions at the particle filter. Such model can lead to a more precise estimation of the soot loading.        In some embodiments, the process may be used to de-poison a NOx reducing device. It may then comprise, prior to the step a), a step of detecting a regeneration trigger which comprises estimating the NOx reducing device is poisoned.        In some embodiments, the preferred first temperature range may be between 450 at 510 degrees Celsius.        In some embodiments; the step of gradually increasing the temperature at the particle filter to a second temperature range may comprise increasing the temperature at the particulate filter up to over 600° C., for example up to around 620 to 625, or even up to 640°.        In some embodiments, the temperature at the particle filter may be maintained within the second rage during a second period of time.        
According to another aspect of the present invention, a process is provided for regenerating an exhaust gas after-treatment device in an exhaust line of an internal combustion engine arrangement, the exhaust line including a particle filter, the process comprising detecting a triggering event indicative of a need for regeneration of the exhaust gas after-treatment device; determining that soot loading of the particle filter has exceeded a safe high temperature regeneration level, setting and maintaining temperature at the particle filter with in a first temperature range during a first period of time until at least one of a predetermined period of time has lapsed, and a determination has been made that soot loading of the particle filter is below the safe high temperature regeneration level; and following the first period of time, increasing the temperature at the particle filter to within a second temperature range above the first temperature range.
According to another aspect of the invention, a process is provided for regenerating an exhaust gas after-treatment device in an exhaust line of an internal combustion engine arrangement, the exhaust line including a particle filter, the process comprising maintaining at the particle filter within a first temperature range during a first period of time;
after the first period of time, increasing the temperature of the exhaust gases at the particle filter to within a second temperature range above the first temperature range,
wherein, following the first period of time, increasing the temperature at the particle filter to within a second temperature range above the first temperature range comprises controlling a rate of increase of the temperature of the exhaust gases at the particle filter from the first temperature range to the second temperature range.
According to another aspect of the invention, an internal combustion engine arrangement is provided, comprising:
an internal combustion engine,
an exhaust line for collecting the exhaust gas from the engine and conducting the exhaust gas towards the atmosphere;
an exhaust after-treatment system in the exhaust line, the exhaust after-treatment system comprising at least a particle filter,
heating means arranged to increase a temperature at the particle filter; and
a controller for controlling the heating means
characterized in that the controller is arranged perform a process as described above.
According to a further aspect of the invention, a vehicle is provided which comprises an internal combustion engine arrangement having any of the above features, and/or arranged for performing a process according as defined above.